Calcijex is used in the treatment of secondary hyperparathyroidism because it suppresses the synthesis and secretion of parathroid hormone. However, because of its potent effects on intestinal calcium absorption and bone calcium mobilization, treatment with Calcijex can induce hypercalcemia, often precluding its use at therapeutic doses. In uremic rats, treatment with Zemplar causes much less hypercalcemia than does treatment with Calcijex because Zemplar is approximately 10 times less effective in mobilizing calcium from the skeleton, compared with Calcijex, This study will determine in humans the relative effects of Zemplar and Calcijex on calcium resorption from bone.